1. Field of the Invention
The present invention relates to a linear motion apparatus of a circulation type such as a ball screw, a linear guide and a ball spline and so on.
2. Description of the Related Art
In a machine tool and an injection molding machine, as a mechanism part thereof for converting the rotational movement of a motor or the like into a linear movement, there is used a ball screw. A ball screw of this type includes a screw shaft and a nut to be movably fitted with the outer periphery of the screw shaft. In case where one of the screw shaft and nut is rotated, a large number of balls incorporated into the nut are allowed to roll between two spiral-shaped ball rolling grooves which are respectively formed in the outer peripheral surface of the screw shaft and in the inner peripheral surface of the nut in such a manner that they are opposed to each other. And, the ball screw of this type further includes a ball return member for returning the balls back to their respective initial positions; and, the balls, which have rolled along a ball rolling passage formed between the two ball rolling grooves of the screw shaft and nut, are allowed to return to the initial positions through a ball return passage formed in the ball return member.
Most of the ball return members of this type, generally, can be produced by bending a tubular-shaped member made of metal into a U-like shape. Therefore, in case where the inside diameter of the ball return member is almost equal to the diameter of the ball, the balls are easy to be clogged in the bent portion of the ball return member. In view of this, normally, the ball return member is formed using a tubular-shaped member having an inside diameter about 1.1 to 1.2 times the diameter of the ball.
However, in case where the ball return member is formed by a tubular-shaped member having an inside diameter about 1.1 to 1.2 times the diameter of the ball, there is produced a stepped portion in the boundary portion between the ball return passage and ball rolling passage, which causes a ball clogging phenomenon that the balls moving from the ball return passage into the ball rolling passage are caught by the stepped portion, thereby raising a possibility that the proper operation of the ball screw can be greatly impeded. In order to solve this drawback, in JP-B-57-38829, there is disclosed a ball screw in which, in the portion of the ball rolling groove of the nut that extends over a proper range from the two ends of the ball return member, there is formed an escape portion which increases gradually toward the leading end of the ball return member.
In the ball screw of the above-cited type, the escape portion makes it difficult to cause such a stepped portion as impedes the rolling movements of the balls in the boundary portion between the ball return passage and ball rolling passage, thereby being able to prevent the ball clogging phenomenon from occurring in the boundary portion between the ball return passage and ball rolling passage. However, in this structure, there is still found the following problem. That is, when such escape portion as disclosed in the above-cited publication is going to be formed in the ball rolling groove of the nut using a working machine such as a ball end mill, the setting of the working machine is very complicated and thus provides a high degree of difficulty, which results in an increase in the manufacturing cost of the ball screw.